This invention relates generally to modulators and modulation methods of the type which convert an analog input to an output pulse train having an average amplitude over time proportional to the analog input.
Circuits for producing modulated pulse train outputs proportional to an analog input have numerous applications. Where high accuracy and a wide dynamic range are required, such as in electrical power metering, it is desirable to employ delta-minus-sigma modulation. Delta-minus-sigma modulation has long been known; see, for example Inose et al., "A Unit Bit Coding Method By Negative Feedback", Proceedings of the IEEE, November, 1963, page 1524. Such modulation employs feedback of the output pulse train, or reference voltages or currents controlled by the output pulse train, to a summing node to which the input signal is also supplied. The difference between the two signals is integrated and compared to a predetermined reference level. When the reference level is crossed, a gate is opened or closed, either beginning or terminating an output pulse and simultaneously changing the signal being fed back to the summing node. The integrated signal is thus always maintained in the vicinity of the reference level and, assuming an input signal varying sufficiently slower than the output pulses, the output pulse train will carry information reflecting the input amplitude to a high degree of accuracy.
For the above system to operate over a wide dynamic range, particularly at low input amplitudes, the measuring circuit internal components must be relatively free of errors. One specific problem is that amplifier circuitry generally has internal voltage offsets which must be compensated. The term voltage offset is generally defined as the voltage difference between the inputs of an active circuit element, such as an operational amplifier, when the output is at zero. Ideally, the voltage offset will be zero. When it is not, offset can easily be corrected for, once its value is known. Offset measurement and corrective calibration are expensive, however, as is the use of low error components, so manufacturers of equipment requiring high accuracy analog modulators generally resort to "zeroing out" strategems.
It would be advantageous to provide a modulator for converting an input signal to a pulse train which has an average amplitude proportional to the magnitude of the input signal, and which has both high accuracy, and a wide dynamic range. It would be particularly advantageous if such a circuit would automatically compensate for internal voltage offsets without the need for expensive calibration or the use of error-free components.